Dynamic networks for recycling thermoset polymers

ABSTRACT

Methods for recycling thermoset polymers, particularly by changing them into dynamic networks with the use of an appropriate catalyst solution which transforms the thermoset polymer into a vitrimer-like composition. The methods include the step of swelling a crosslinked thermoset polymer in a solution including a catalyst, whereby the catalyst diffuses into the thermoset polymer, in particular into the thermoset network. Upon removal of the liquid portion of the solution, such as solvent, the catalyst facilitates the occurrence of exchange reactions at elevated temperatures, rendering the system a dynamic network. The vitrimerized composition having the thermoset polymer and catalyst is recyclable and processable and thus suitable for many end uses.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract 1243313awarded by the National Science Foundation. The Government has certainrights in the invention.

FIELD OF THE INVENTION

The present invention relates to methods for recycling thermosetpolymers, particularly by changing them into dynamic networks with theuse of an appropriate catalyst solution which transforms the thermosetpolymer into a vitrimer-like composition. The methods include the stepof swelling a crosslinked thermoset polymer in a solution including acatalyst, whereby the catalyst diffuses into the thermoset polymer, inparticular into the thermoset network. Upon removal of the liquidportion of the solution, such as solvent, the catalyst facilitates theoccurrence of exchange reactions at elevated temperatures, rendering thesystem a dynamic network. The vitrimerized composition comprising thethermoset polymer and catalyst is recyclable and healable and thussuitable for many end uses.

BACKGROUND OF THE INVENTION

Thermosets are covalently cross-linked networks that, unlikethermoplastics cannot be reprocessed by melting or dissolution in anysolvents. These networks can exhibit a glass transition temperature(T_(g)) lower than the designed application service temperature(elastomer/rubber) or higher than the service temperature (thermosetresins). Thermoset rubbers find applications in the automotive industry(interiors, bumpers), biomedical devices, bedding, furniture, packaging,gaskets, O-rings and so on.

Usually thermoset resins exhibit significant benefits in comparison withthermoplastics exhibiting dimensional stability, high mechanicalproperties, high thermal/creep/and chemical resistance, durability. Thisclass of polymers maintain their structural strength, thermal andelectrical resistance characteristics during prolonged use. Thermosetsfind many industrial uses as coatings, adhesives but also in fiberreinforced Thermosets find many industrial uses as coatings, adhesivesbut also in fiber reinforced composites for many high-tech applications.Composites high stiffness and strength in combination with their lightweight make them play a vital role in the clean energy production (e.g.wind turbine blades, hydrokinetic power generation, support structuresfor solar systems and their encapsulations, and geothermal energyproduction) manufacturing of lighter vehicles (automotive, airplanes,trains, boats and aerospace) for reduced fuel consumption. There aremany other applications requiring high structural strength anddurability, thermal and corrosion resistance, such as structuralmaterials for buildings, pipelines, industrial equipment and/or theircomponents for instance heat exchangers, light-emitting diode lenses,fly-wheels for electricity grid stability, containers, or off-shorestructures in which composite materials are the material of choice.

Despite the great features of thermosets, they are typically produced inrelatively small quantities and are expensive. Their main advantage,which is retaining a lasting and intractable three-dimensionalstructure, creates also a crucial disadvantage since they cannot berecycled or reprocessed.

A very interesting strategy to induce re-formability and healing inchemically cross-linked polymer networks is by using exchangeablechemical bonds that will lead to a dynamic cross-linked network.Polymeric systems containing such exchangeable bonds are covalentadaptable networks (CANs). Depending on the exchange mechanism, CANs canbe classified to two different categories. The first category comprisesnetworks where the exchange mechanism of crosslinks is dissociative. Forsuch networks, most of the cross-links break under certain conditions(temperature, UV-light exposure, pH,) and re-form again with a change inthe conditions. These type of adaptive networks show a sudden andsignificant decrease in the viscosity, with breaking the cross-linkingbonds.

In the second category, the mechanism of crosslinking is associative anda crosslinking bond does not break until a new bond forms, which makesthe network permanent and dynamic.

Vitrimers are polymeric associative CANs that have permanent networksand demonstrate a gradual viscosity decrease upon heating, which is adistinctive character of vitreous silica. By definition, vitrimers arepolymeric networks made with covalent crosslinking. The crosslinkingbonds of such networks have an associative nature which results in theability of material to change its topology via exchange reactions. Thesereactions are triggered by heat which causes a gradual decrease insystem viscosity with increasing temperature and provides malleabilityto the network. The viscosity of vitrimers is governed by the chemicalexchange reaction at elevated temperatures and like silica and unlikedissociative networks and thermoplastics, decreases gradually. Vitrimersmaintain permanent network at all temperatures until degradation, andthey can swell but not dissolve in specific solvents. However, swellingratios are higher for these networks in comparison with the non-dynamicones.

Due to importance of polymer recycling, scientists are interested in theconcept of designing materials based on cradle-to-cradle life cycle.Dynamic networks offer the opportunity to design materials consideringthe cradle-to-cradle concept. However, such systems do not exhibitproperties at par with thermosets and are sometimes prone to earlydegradation. Application of such networks is restricted at hightemperatures.

SUMMARY OF THE INVENTION

Reprocessing of existing thermoset waste is presented. Unrecyclablethermoset materials are reprocessed using associative dynamic bonding.The developed recycling methodology relies on swelling the thermosetnetwork into a solution of a catalyst which enables transesterificationreactions characteristic of vitrimer chemistry allowing dynamic bondexchange between ester and hydroxyl groups within the thermoset network.Thermal and mechanical properties for recycled polyurethane and epoxynetworks are described along with a strategy to maintain the propertiesof recycled materials. The developed methodology promises recycling andreprocessing of previously thought intractable materials. Moreover,processability of vitrimerized thermosets with common thermoplasticmanufacturing methods opens up the possibility of tuning recyclednetworks by adding nanoparticles. This flexibility keeps the applicationwindow of recycled thermosets very broad.

In view of the above, the present invention provides recyclablethermoset polymers and methods for producing recyclable thermosetpolymers.

Accordingly, one aspect of the present invention relates to avitrimer-like composition, comprising a crosslinked polymer and acatalyst infused in a network comprising the crosslinked polymer,wherein the composition is a dynamic network.

A further aspect of the invention relates to a method for producing arecyclable thermoset composition, comprising the steps of: obtaining acrosslinked polymer; contacting the crosslinked polymer with a solutioncomprising a catalyst and a liquid such that the catalyst becomesinfused in at least a portion of the crosslinked polymer; and removingthe liquid from the crosslinked polymer and solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features andadvantages will become apparent by reading the detailed description ofthe invention, taken together with the drawings, wherein:

FIG. 1 illustrates, left side, polyurethane pieces and vitrimerizedpolyurethane pieces comprising a crosslinked polymer and a catalyst tothe immediate right thereof, wherein the vitrimerized compositionexhibits tinges of yellow due to incorporation of the catalyst and,right side, an image of the respective pieces after pressing at 180° C.for 20 minutes, wherein it is clearly observed that the untreatedpolyurethane is in a powder-like form and the vitrimerized compositionhas formed a film;

FIG. 2 illustrates, left side, mechanical performance of recycled(vitrimerized) polyurethane and original, untreated polyurethane and, onthe right, the exceptional extensibility of vitrimerized polyurethane isshown; and

FIG. 3 illustrates, left side, vitrimerized epoxy pieces comprising acrosslinked polymer and a catalyst after solvent penetration, rightside, an image of the pieces after pressing at 250° C. for 20 minutes,wherein it is clearly observed that the vitrimerized composition hasformed a film.

FIG. 4 is an overview of the process wherein a permanent thermosetnetwork can be extruded, injection molded and compression molded aftervitrimerization.

FIGS. 5(a) and (b) illustrates representative tensile curves for theepoxy and PU system respectively. (b) also shows the effect of a secondrecycling on the mechanical properties (c) stress relaxation behaviorfor the neat, vitrimerized and vitrimer epoxy system. (d) illustratesthermal stability and decomposition behavior of samples.

FIG. 6 illustrates (a) vitrimerized PU with 10 wt % CNS hot pressed intoa thin film (b) vitrimerized PU with 10 wt % CNS extruded into strandsand (c) representative tensile curves of neat and recycled PU/CNSsystems.

FIG. 7 illustrates the effect of multiple recycling cycles on thermaldecomposition temperatures of a material.

DETAILED DESCRIPTION OF THE INVENTION

Methods for recycling crosslinked thermoset polymers are disclosedherein. The method recycles the thermosets by changing them into dynamicnetworks with the use of an appropriate catalyst solution which resultsin a vitrimer-like composition. The catalyst facilitates exchangereactions which can take place at elevated temperatures, rendering thevitrimerized composition system a dynamic network. Importantly, thecrosslinked thermoset polymer becomes recyclable and healable utilizingthe methods of the present invention.

Through the present invention, a thermoset material is provided that isable to be used for its intended application and thereafter can beconverted into a vitrimer-like form that can be utilized in a secondapplication.

Nanoparticles are utilized in some embodiments in order to tune theproperties of the recycled thermosets.

Thermoset Polymers

The present invention includes at least one crosslinked thermosetpolymer in order to form a vitrimer composition. Various crosslinkedthermoset polymers can be utilized. For example, crosslinked thermosetpolymers include, but are not limited to, polyester based polyurethane,epoxy, polyesters and any crosslinked network which has ester groups onthe backbone of the polymer as well as available OH groups on thestructure.

Catalysts

The vitrimer-like compositions of the present invention also include acatalyst that has the ability to facilitate exchange reactions of thecrosslinked thermoset polymer. The catalyst must also be able to diffuseinto the network of the crosslinked thermoset polymer.

Generally, the catalyst can be chosen from metal salts of zinc, tin,magnesium, cobalt, calcium, titanium and zirconium. A catalyst withlower activation energy is preferred since it leads to fastertransesterification kinetics. The catalyst may also be chosen fromcatalysts of organic nature, such as but not limited to,benzyldimethylamide, and benzyltrimethylammonium chloride. The catalystshould be chosen such that it should be soluble in the liquid or solventwhich is used for polymer swelling and have a sufficiently highdegradation temperature.

That said, suitable catalysts include tin(II) 2-ethylhexanoate,1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), zinc(II)acetate (Zn(OAc)₂),triphenylphosphine (PPh₃), dibutyltin bis(2-ethylhexanoate), dibutyltindiacetate, dibutyltin dilaurate, dibutyltin bis(2,4-pentanedionate),titanium 2-ethylhexanoate, monobutyltin oxide, and zinc octoate.

In one embodiment of the invention, the catalyst is utilized in anamount sufficient to produce a vitrimer having desired properties. Forexample, in one embodiment, at least 2 wt. % catalyst based on thethermoset polymer weight is utilized.

A catalyst concentration of about 10 wt. % in a suitable solvent ispreferred in some embodiments.

Catalyst Solution Liquid

As described herein, the catalyst is placed in a liquid to form acatalyst solution and the solution is contacted with the crosslinkedthermoset polymer in order to infuse the catalyst into the polymer. Theliquid utilized preferably has one or more of the followingcharacteristics: the ability to dissolve the catalyst, and the abilityto swell the polymer to allow infusion of the catalyst into the polymernetwork. Preferably, the liquid has a low evaporation temperature. Insome embodiments the catalyst solution liquid comprises 10 wt. % or 20wt. % or more of the catalyst. However, lower amounts can be utilized inother embodiments.

Examples of suitable liquids include, but are not limited to,dichloromethane (DCM), Tetrahydrofuran (THF), N,N-Dimethylformamide(DMF), and Dimethylsulfoxide (DMSO). Basically, the liquid, i.e. solventshould be able to dissolve the catalyst being used as well as swell thepolymer. Thus, the solvent of choice can change with the polymer typeand catalyst.

Nanoparticles

In various embodiments of the present invention, the compositions ofrecycled thermosets can have nanoparticles incorporated therein. Carbonnanostructures are preferred in one embodiment. Similar nanoparticleslike such as zinc oxide, titanium dioxide, nanoclay, carbon black,graphene oxide can be used. Mixtures of nanoparticles are contemplated.The nanoparticles, when utilized, are preset in a weight concentrationup to about 30 wt % based on the total weight of the composition. Micronsized fillers can be used as well. Any filler that can be compoundedusing conventional mixing elements like an extruder can be incorporated.

Examples

Vitrimerized Polyurethane

Materials

Crosslinked polyurethane (PU) was synthesized by polymerizingpolycaprolactone triol (Mw=900 g/mol) with 1,4-phenylene diisocyanate(PPDI). 10 g of polyol was degassed under vacuum at 80° C. overnightthen transferred into a sealed three neck reactor under nitrogen and 40ml of DCM was added to the reactor and stirred to dissolve the polyol.The PPDI was mixed with the same amount of DCM. The PPDI solution wasadded to the polyol solution at 40° C. and stirred rigorously for 3mins. The mixture was cast on a Teflon® plate and the solvent (DCM) waslet to evaporate at room temperature under nitrogen flow for 48 hours.The resulting films were placed in vacuum oven at 80° C. for 24 hours toeliminate any traces of solvent from the PU.

Crosslinked PU was cut into small pieces and added to a solution of20-40 wt % tin(II) 2-ethylhexanoate (Sn (Oct)₂) in dichloromethane(DCM). The mixture was stirred for 72 hours to ensure that the PU wascompletely swollen and the catalyst has infused into the PU network. PUpieces were separated from solution by sieving and then placed in vacuumoven for 24 hours at 25° C. PU pieces were then washed with isopropylalcohol in order to remove any remaining catalyst on the PU surfaces andthen were placed in vacuum oven for another 24 hrs at 80° C. to removeany solvent from the system. The dried PU pieces were placed betweenTeflon sheets and pressed at 180° C. for 20 mins and formed a cleardefect free film. Dog-bone samples were cut from this film and were usedfor mechanical analysis.

Methods

Uniaxial tensile tests were carried out at room temperature with aZwick/Roell (Model Z0.5) with load cell of 100N on at least 4 samples atan extension rate of 10 mm/min. Samples had dumbbell shape and were cutfrom films of PU which were prepared by compression molding.

Results

In order to prove the concept that we are able to recycle crosslinked PUrubber, we attempt to reshape small cut pieces into a film using hotpress. As FIG. 1 shows, original PU pieces are transparent in color andthe recycled PU has tinges of yellow due to incorporation of thecatalyst. The color change proves that we have successfully infuse thecatalyst into the PU network. After the hot press we observe that theuntreated PU breaks into powder whereas the recycled/treated PU flowsand forms a good quality film.

The mechanical performance of recycled PU was investigated and comparedto the mechanical properties of neat PU. Those properties are shown inFIG. 2 and summarized in table 1. It can be seen that after recycling32% of modulus and 57% of ultimate strength are recovered andinterestingly the extensibility of PU (elongation at break) hasincreased by 92%. Significant extensibility of the recycled PU is alsoseen in FIG. 2 which shows that the recycled PU can extend more than 5times of its original length. Results obtained from the mechanicalanalysis show that recycling is effective and successful.

TABLE 1 Results obtained from tensile experiments Sample name Modulus(MPa) Strength (MPa) Elongation at break (%) PU 5.637 ± 0.48 10.7 ± 2.79270 ± 31 Recycled PU 1.811 ± 0.14 6.13 520

Vitrimerized Epoxy 1

The epoxy DGEBA (diglycidyl ether of Bisphenol-A) and both the catalyststin(II) 2-ethylhexanoate and zinc acetylacetanoate were purchased fromSigma Aldrich and used as received. The fatty acid (Pripol™ 1040) wasobtained from Croda Inc. and used as received.

The transesterification reaction that we are exploiting in this workrequires the presence of hydroxyl and ester groups. Hydroxyl groups arereadily available in any epoxy sample as ring opening of the oxiranemolecule results in the formation of hydroxyl groups. The choice ofhardener dictates the presence or absence of the ester groups. To thiseffect, we chose fatty acid curing agents which result in ester groupsbeing formed upon curing.

DGEBA and Pripol™ 1040 were hand mixed together using a glass rod in theweight ratio of epoxy:hardener=1:1.7 (the stoichiometry between theepoxy group to COOH is 1:1). The premixed samples were poured into asilicon mold and cured in two stages. The mixture was first cured at 80°C. until gel and 160° C. for 6 hours. After the samples were completelycured, they were cut into small pieces with dimensions of approximately2 mm*2 mm*3 mm.

The catalyst tin(II) 2-ethylhexanoate which is a liquid at roomtemperature was dissolved in 3 solvents namely dimethylformamide (DMF),tetrahydrofuran (THF) and dichloromethane (DCM). 28 wt % of thiscatalyst solution was prepared in ˜18 ml of solvent. Approximately200-300 mg of the cut epoxy sample pieces were then immersed into eachof these catalyst solvent solutions and left to stir at room temperaturefor 72 hours. 20 wt. % of the catalyst zinc acetylacetonate which is inthe form of powder at room temperature was melted and dissolved in ˜18ml of DMF at 130° C. 200-300 mg of the cut epoxy samples were immersedinto this catalyst solution and left in the oven at 130° C. for 72hours.

After 72 hours, the 4 different samples were dried at 80° C. for 48hours following which, the dried samples were washed with ethanol andthen dried for another 12 hours at 80° C. The washed and dried epoxysample pieces were then hot pressed at 250° C. for 20 minutes. A controlsample that was not immersed into any catalyst solution was also hotpressed at 250° C. for 20 minutes.

Samples without any catalyst penetration i.e. the control sampleremained as discrete pieces irrespective of hot pressing. All thesamples which underwent the above mentioned process of catalystpenetration formed uniform thin films.

Vitrimerized Epoxy 2

Epoxy networks were synthesized using DGEBA (Diglycidyl ether ofBisphenol A) and Glutaric anhydride. The reactants were hand mixedtogether using a glass rod such that the ratio of [epoxygroups]:[anhydride groups] is 1:1. An imidazole catalyst was added at 1wt % with respect to DGEBA to accelerate the reaction. The mixture waspoured into a mold and cured at 80° C. and then 160° C. for 8 hourseach.

Tensile tests for the epoxy system were conducted on a Zwick/Roell witha load cell of 500N at an extension rate of 1 mm/min. Samples wererectangular shaped and were obtained from the minilab in a process thatwill be explained later herein.

TGA:

Thermal decomposition tests were run on samples on a Q500 TA instrumentsmachine under N2 flow. Samples were ramped upto 700° C. at 10° C. perminute.

Stress Relaxation:

The Ares G2 Rheometer was used to measure the stress relaxation behaviorof all samples. Samples were studied in parallel plate geometry at atemperature of 200° C. Samples were subjected to a strain percentagefollowing which the relaxation in storage modulus was observed withtime. Care was taken to ensure that the strain values were within theviscoelastic regimes of all samples.

Results and Discussion

The transesterification reaction was selected as a model chemistry forvitrimerization in this research because of its importance and abundancein many thermoset systems. Any thermoset material that has ester groupsand freely available hydroxyl groups can be infused with a catalyst thatsupports transesterification and thereby be converted into avitrimer-like material. In this research the effect of vitrimerizationon recyclability of epoxy and polyurethane as two widely used thermosetmaterials has been shown.

The catalyst employed in the examples, Tin (II) 2-ethylhexanoate(Sn(Oct)₂) was chosen for its high catalytic activity intransesterification reactions as well as its high stability in mostmedia.

The Vitrimerization Process:

The infusion of catalyst into the sample to make a vitrimerized materialis also dependent on temperature. The infusion was much more successfulif it was carried out at a temperature above the Tg of the sample. It isbelieved that the solvent swelling at temperatures higher than network'sTg facilitated the diffusion of the catalyst molecules into thethermoset network. FIG. 4 shows a schematic of the vitrimerizationprocess. In general, a 10 wt % catalyst solution (solvent varied basedon temperature of infusion) was prepared, and small pieces of thethermoset samples (approximately 2 mm*2 mm*2 mm) were immersed into thesolution for 48 hours. The samples were then washed with ethanol toremove the catalyst from the surface and dried in a vacuum oven for12-24 hours. More specifically, the infusion for PU (having a Tg of ˜−7°C.) was successful at room temperature using Dichloromethane (DCM) asthe solvent. The samples after infusion and washing with ethanol weredried at 80° C. for 24 hours. The epoxy sample which had a Tg of 95° C.was infused with the catalyst at 140° C., since its glassy behavior atroom temperature would prevent significant catalyst diffusion.

Logically, a high temperature boiling point solvent Dimethylformamide(DMF) replaced DCM for the hard network infusion. The infusion wascarried out for 48 hours, following which the samples were washed withethanol and dried at 140° C. in a vacuum oven for 24 hours.

Processing of the Vitrimerized Samples:

The infused or ‘vitrimerized’ samples were processed by compressionmolding and extrusion. FIG. 4 shows samples made from each of theseprocesses. In the case of compression molding, preheating time, pressingtime and temperature played a role in determining final film quality. PUwas processed at 200° C. with 5 minutes of preheating time and 5 minutesof pressing at a pressure of 5 metric tons. A higher temperatureresulted in the onset of degradation of the PU material. The epoxysamples were hot pressed at 250° C. with 20 minutes of preheating timeand then pressed for 15 minutes at a pressure of 5 metric tons.Temperatures less than 250° C. resulted in incomplete films.

Extruded samples were obtained by feeding the vitrimerized material intothe hopper of a twin screw counter rotating minilab operating at atemperature of 205° C. and at an RPM of 15.

Besides extrusion, some samples in the minilab were also forced into amold to form rectangular shaped samples as can be seen in FIG. 4, aprocess akin to injection molding.

Properties of the Vitrimerized Samples:

Dog bone shaped PU thin films obtained from hot pressing were comparedfor mechanical testing with their neat counterparts, i.e. PU sampleswhich were not vitrimerized. Rectangular shaped samples obtained fromthe minilab were used for mechanical testing in the case of the epoxysystem. Those properties are shown in FIGS. 5(a) and (b) and summarizedin the tables below.

TABLE a Mechanical properties comparing neat, vitrimer and vitrimerizedsamples for PU system Strength Elongation at Samples (MPa) Modulus (MPa)break (%) Neat 7.84 ± 0.85 5.56 ± 0.46   246 ± 37.1 Vitrimerized 1^(st)2.35 ± 0.22 2.43 ± 0.06   192 ± 16.9 recycle Vitrimer 1 wt %  2.46 ±0.277 2.41 ± 0.16 195 ± 19 Vitrimer 5 wt %  2.37 ± 0.107 1.791 ± 0.100261 ± 40 Vitrimer 10 wt % 3.227 ± 0.223 2.481 ± 0.063 241 ± 14

TABLE b Mechanical properties comparing neat, vitrimer and vitrimerizedsamples for epoxy system Elongation at break Samples Strength (MPa)Modulus (MPa) (%) Neat   40 ± 1.41 802 ± 100.41  8 ± 0.1 Vitrimerized1^(st) 26 600 6.8 recycle Vitrimer 5 wt % 22.13 ± 5.2  918 ± 11.16  3.8± 0.03

TABLE c Mechanical properties of a PU system with multiple recycling andaddition of nanofillers Elongation at break Samples Strength (MPa)Modulus (MPa) (%) Neat 7.84 ± 0.84 5.55 ± 0.45 246 ± 37.1 Recycled(1^(st)) 2.35 ± 0.22 2.43 ± 0.06 192 ± 16.9 Recycled (2^(nd)) 2.10 ±0.22 1.22 ± 0.22 259 ± 52   Recycled 7.251 ± 1.532 13.48 ± 2.88  67.5 ±28.6  10 wt. % CNS

It can be seen that after recycling, 44% of modulus, 30% of ultimatestrength and 78% of extensibility for PU could be recovered. In the caseof the epoxy system, 73.5% of modulus, 63% of ultimate strength and 82%of extensibility were recovered.

The effect of multiple recycling on vitrimerized samples was alsoinvestigated as can be seen in FIG. 5(b) and FIG. 7. Previously thoughtto be unrecyclable PU waste, the vitrimerized samples could now bereprocessed up to 2 times. The third recycling resulted in a pasteunable to form a film of any substantial strength. In the 2^(nd)recycling process there was a 10% decrease in ultimate tensile strengthand a 50% decrease in modulus, while the elongation at break increasedby 35% by comparison with the first recycle. The multiple recycling didnot however have any effect of the thermal decomposition temperatures ofthe material as is observed in FIG. 7.

Since vitrimerization also allows for the processing of a thermoset justlike a thermoplastic, it affords the unique advantage of incorporatingfillers into the thermoset network during recycling to further increasemechanical properties.

Nanoparticle-Containing Example

10 wt % of highly branched carbon nanotubes with a diameter of 7-9 nmknown as Carbon Nanostructures (CNS) were extruded along with thevitrimerized PU samples with the same parameters mentioned above andtested for mechanical properties as can be seen in FIG. 6(c). Thenanocomposite exhibited brittle behavior showing an equivalent ultimatetensile strength to that of the neat sample and a modulus that was ˜2.5times higher.

We have clearly shown that PU and epoxy thermoset materials, previouslythought to be waste and a burden to the environment past their intendeduse can be recycled and reprocessed once more via the vitrimerizationprocess. Conventional processing techniques of compression molding,extrusion and injection molding have been described herein. We havefurther shown that during the reprocessing, incorporation of suitablefillers like Carbon Nanostructures (CNS) may significantly improve theproperties to resemble or even surpass those of the original material.We have taken advantage of the transesterification chemistry for dynamicbonding. With the right catalyst or a combination of differentcatalysts, this work can be extended to other vitrimer chemistries.

Formation of thin films has been illustrated herewith. Other processingmethods including extrusion, injection molding, and 3-D printing can beutilized as well.

In accordance with the patent statutes, the best mode and preferredembodiment have been set forth; the scope of the invention is notlimited thereto, but rather by the scope of the attached claims.

What is claimed is:
 1. A vitrimer-like composition, comprising: acrosslinked polymer and a catalyst infused in a network comprising thecrosslinked polymer, wherein the composition is a dynamic network. 2.The composition according to claim 1, wherein the crosslinked polymercomprises polyurethane, and wherein the catalyst comprises tin(II)2-ethylhexanoate.
 3. The composition according to claim 2, wherein thepolyurethane is derived from a composition comprising polycaprolactonetriol and 1,4-phenylene diisocyanate.
 4. The composition according toclaim 1, wherein the crosslinked polymer comprises epoxy and a curingagent.
 5. The composition according to claim 4, wherein the epoxycomprises diglycidyl ether of Bisphenol-A and the curing agent comprisesa fatty acid.
 6. The composition according to claim 5, wherein thecatalyst comprises tin(II) 2-ethylhexanoate or zinc acetylacetonate. 7.A composition according to claim 1, wherein the composition furtherincludes nanoparticles.
 8. A composition according to claim 7, whereinthe nanoparticles are present in an amount from about 1 to about 30parts by weight based on the total weight of the composition.
 9. Acomposition according to claim 8, wherein the nanoparticles comprisecarbon nanotubes.
 10. A method for producing a recyclable thermosetcomposition, comprising the steps of: obtaining a crosslinked polymer;contacting the crosslinked polymer with a solution comprising a catalystand a liquid such that the catalyst becomes infused in at least aportion of the crosslinked polymer; and removing the liquid from thecrosslinked polymer and solution.
 11. The method according to claim 7,wherein the crosslinked polymer comprises one or more of polyurethaneand epoxy.
 12. The method according to claim 8, wherein the catalystcomprises tin(II) 2-ethylhexanoate or zinc acetylacetonate.
 13. Themethod according to claim 9, wherein the polyurethane is derived from acomposition comprising polycaprolactone triol and 1,4-phenylenediisocyanate, and wherein the epoxy network is derived from diglycidylether of Bisphenol-A and a fatty acid.
 14. The method according to claim7, wherein the contacting step comprises placing the crosslinked polymerin the solution.
 15. The method according to claim 11, wherein thecrosslinked polymer is contacted for a sufficient amount of time toswell the crosslinked polymer.
 16. The method according to claim 12,wherein removing the liquid from the crosslinked polymer and solutioncomprises placing the crosslinked polymer in a vacuum oven at 80° C. forat least 12 hours.
 17. The method according to claim 10, furtherincluding the step of incorporating nanoparticles into a compositioncomprising the crosslinked polymer after the liquid is removed from thecrosslinked polymer and solution.
 18. The method according to claim 17,wherein the nanoparticles are present in an amount from about 1 to about30 parts by weight based on the total weight of the composition.
 19. Themethod according to claim 18, wherein the nanoparticles comprise carbonnanotubes.